The present invention relates generally to a manner by which to overcome antenna feeder cable loss at a receiving station, such as a radio base station of a cellular communication system. More particularly, the present invention relates to apparatus and an associated method, by which a receive signal, received at the receiving station, is amplified to more than overcome the effects of the cable loss. Thereafter, the signal is selectively attenuated, to thereby to provide a receive signal, with a selected amount of gain, to the receiver circuitry. Operation of an embodiment of the present invention provides a receive signal to the receiver circuitry in a manner which exhibits a low noise figure and high linearity.
A communication system is formed, at a minimum, by a transmitting station and a receiving station interconnected by a communication channel. Information to be communicated by the transmitting station to the receiving station is converted into a form to permit its transmission upon the communication channel. Once converted into the form to permit its transmission upon the communication channel, a communication signal, formed at the transmitting station, is transmitted upon the communication channel to be received at the receiving station. Once received at the receiving station, the informational content of the signal is recovered.
A radio communication system is a type of communication system in which the communication channel is formed of a radio channel defined upon a portion of the electromagnetic spectrum. Because a fixed connection is not required to form the communication channel between the transmitting and receiving stations, communications can be effectuated pursuant to the radio communication system when a fixed connection between the transmitting and receiving stations would otherwise be impractical.
A cellular communication system is exemplary of a radio communication system. A cellular communication system is further a multi-user communication system which permits concurrent communication by a plurality of different users. A cellular communication system includes network infrastructure with which mobile stations communicate by way of radio channels. The network infrastructure includes a plurality of spaced-apart radio transceivers, referred to as radio base stations, positioned at selected locations throughout a geographical area to be encompassed by the cellular communication system.
Communication is effectuated between a radio base station of the network infrastructure and a mobile station positioned proximate thereto. Transmitter circuitry of the mobile station modulates the information which is to be communicated to the base station by modulating the information upon a radio frequency signal. Typically, the signals generated by a mobile station are of relatively low power levels, mostly less than three watts, and usually six hundred milliwatts. As the electromagnetic spectrum is not a noise-free communication medium, noise and other interfering signals are also sometimes introduced upon the communication signal transmitted by the mobile station upon a radio channel.
The radio base station must be able to detect the transmitted signal, subsequent to propagation upon the radio channel. As the transmitted signal is propagated upon the radio channel, the signal is susceptible to the effects of noise and other interfering signals, as well as normal signal attenuation.
A radio base station typically includes one or more antenna transducer elements operable to detect the electromagnetic signals transmitted thereto by a mobile station. An antenna transducer element converts the transmitted signals from electromagnetic form into electrical form. To detect better the electromagnetic signals, conventional techniques are typically utilized, including, for instance, positioning the antenna transducer at an elevated position. By elevating the position of the antenna transducer, physical obstructions are less likely to obstruct, or otherwise interfere, with the transmission of the signal to the radio base station upon the radio channel. The antenna transducer, and associated elements, commonly referred to as a mast head, is coupled to other circuitry of the radio base station by way of a cable, such as a coaxial cable.
However, cable loss is typically associated with the cable. That is to say, when a signal, received by the mast head and transduced into electrical form, is conducted through the cable to other circuitry of the radio base station, the electrical signal is attenuated by an amount corresponding to, and defined by, the cable loss.
Conventionally, in an attempt to overcome such cable loss, an amplifier, referred to as a mast head amplifier (MHA) is positioned at the mast head to amplify the electrical signal prior to its conduction through the cable. The amplification level conventionally selected corresponds to the expected level of cable loss. Thereby, subsequent to conduction of the signal through the cable, the signal level of the signal, when applied to other circuitry of the radio base station, is restored to its original level, i.e., the level of the signal prior to its conduction through the cable. Amplification of the signal with a mast head amplifier, however, also amplifies non-signal components of the signal. That is, noise is also amplified by the mast head amplifier.
For instance, if the amplification of the mast head amplifier is about 5.0 decibels, and a noise figure is of 2.5 dB, and the cable loss is of a corresponding 5.0 decibel level, a zero decibel gain results, but the 2.5 decibel noise figure degrades the sensitivity of the signal applied to the other portions of the radio base station. The normal receiver noise figure contribution is not decreased because there is no net gain from the mast head amplifier and antenna cable.
To improve the noise figure and sensitivity, the gain of the mast head amplifier can be adjusted to be greater than the cable loss, and the front end of the radio base station exhibits a net gain. The resultant effect of the increased amplifier gain is to reduce the resultant noise figure. And, as a result, improved sensitivity results. This type of arrangement is sometimes referred to as an xe2x80x9cover gain principle.xe2x80x9d
This type of arrangement, however, also suffers from disadvantages. While the total noise figure is reduced, the signal and interference levels provided to the other portions of the radio base station are increased. Subsequent to conduction through the cable, the signal is typically applied to a low noise amplifier (LNA). Low noise amplifiers conventionally exhibit relatively low third order intermodulation intercept points. And, such amplifiers generally are constructed to be compatible with signal levels of signals directly available from a mast head. The third order intermodulation intercept point describes the level of interference that two signals can generate to fall at a certain frequency. In practice, the certain frequency is thereafter not useful in the presence of such interference. The two signals together form intermodulation products that, in a cellular communication system, can cause dropped calls. The original signal is typically of a much smaller magnitude than the intermodulation product and the network infrastructure of the communication system is unable to cause the mobile station to generate signals on another channel in time to prevent the dropping of the call.
Another disadvantage of this arrangement is that the RSSI (receiver signal strength indicator) of the receiver portion of the radio base station also becomes inoperable. That is to say, the RSSI operation is affected by the positioning of the external amplifier at the front end of the radio base station.
An improved manner by which to overcome the problems associated with cable loss exhibited at a receiving station, such as a radio base station, would therefore be advantageous.
It is in light of this background information related to receiving stations operable in a radio communication system, that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provide apparatus and an associated method, by which to provide a receive signal of improved noise and linearity to receiver circuitry of the receiving station.
Operation of an embodiment of the present invention overcomes the effects of antenna feeder cable loss at a receiving station, such as a radio base station of a cellular communication system.
In one aspect of the present invention, a mast head amplifier is positioned to amplify electrical signals representative of a receive signal detected by the mast head. The amplification level of the mast head amplifier is selected to be significantly greater than the cable loss exhibited by a cable extending between the mast head and the receiver circuitry of the radio base station. Subsequent to amplification of the signal at the mast head amplifier, the signal is conducted through the cable extending through the mast head and the receiver circuitry of the radio base station. The cable loss associated with the cable reduces the signal levels of the signals conducted therethrough. However, because the amplification level of the mast head amplifier is selected to be greater than the cable loss associated with the cable, the signal, subsequent to conduction through the cable is larger than when applied to the mast head amplifier. An attenuator is positioned between the cable and the receiver circuitry of the radio base station. The attenuator selectively attenuates the signal, subsequent to its conduction through the cable and prior to its application to the receiver circuitry of the base station. The attenuation is selected, for instance, to provide the signal to the receiver circuitry of the base station with a selected level of gain.
In one implementation, the level of attenuation is dynamically determined, that is to say, the level of attenuation is determined responsive to the values of the signals applied to the attenuator. For instance, a cable attenuation detector, in one implementation, is used to detect the cable loss exhibited by the cable and to provide indications of detections made thereat to the attenuator. The attenuation of the attenuator is, in part, responsive thereto.
In another implementation, determinations are made of at least relative signal quality indications, and the level of attenuation of the attenuator is responsive, in part, to such determinations.
Thereby, through operation of an embodiment of the present invention, improved noise figure characteristics and linearity characteristics are possible relative to conventional receiving stations. By providing a high level of amplification at the mast head amplifier and selectively attenuating the signal prior to its application to receiver circuitry of the radio base station, an improved noise figure is provided while also maintaining the linearity of the receiver system and maintaining low distortion of the received signals.
In these and other aspects, therefore, apparatus, and an associated method, selectively modifies characteristics of the receive signal received at a radio base station. The radio base station has a mast head coupled to receiver circuitry by way of a cable. A cable loss is associated with the cable, and a receive signal exhibits a loss corresponding to the cable loss when the receive signal is conducted through the cable. A mast head amplifier is coupled in-line between the mast head and the cable. The mast head amplifier amplifies the receive signal at a mast head level. The mast head amplification is of a value of gain greater than the cable loss. An attenuator is coupled in-line between the cable and the receiver circuitry. The attenuator selectively attenuates the receive signal prior to application to the receiver circuitry.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings which are briefly summarized below, the following detailed description of the presently-preferred embodiments of the invention, and the appended claims.